This invention relates to a dihydrate of calcium hypochlorite having a novel shape and to a method for the manufacture thereof and also to a method for the manufacture of a coarse dihydrate of calcium hypochlorite which is carried out with the dihydrate of the novel shape employed as seed crystal. More particularly, the invention relates to a dihydrate of calcium hypochlorite having a prismatic crystal shape in which the lengths of axes a, b and c thereof are in the ratios of: EQU 0.5.ltoreq.b/a.ltoreq.2.0 EQU c/a.gtoreq.1.5
and the length of the axis c is at least 5 microns and to a method wherein the prismatic dihydrate of calcium hypochlorite having the lengths of its axes a, b and c in the ratios of: EQU 0.5.ltoreq.b/a.ltoreq.2.0 EQU c/a.gtoreq.1.5
and the length of the axis c thereof is at least 5 microns is manufactured by allowing crystallization of the dihydrate of calcium hypochlorite to take place in the presence of one or more than one kind of crystallization medium selected out of a group consisting of carboxylic acid, a carboxylic acid salt and a carbohydrate. The invention also relates to another method wherein a coarse dihydrate of calcium hypochlorite is manufactured by carrying out crystallization of the dihydrate of calcium hypochlorite with the prismatic dihydrate of calcium hypochlorite which has the lengths of its axes a, b and c in the ratios of EQU 0.5.ltoreq.b/a.ltoreq.2.0 EQU c/a.gtoreq.1.5
and the length of the axis c is at least 5 microns being employed as seed crystal and by allowing the seed crystal to grow.